Tricks for how to calculate math percentages

Written by karen adams

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These shoes are discounted, but what is the sale price after subtracting the percentage? (Martin Poole/Photodisc/Getty Images)

A per cent is a ratio of the number to 100 and often is expressed by a "%" symbol. Both "15 percent" and "15%" mean the same and stand for a ratio of "15:100." Percentages are useful in everyday calculations. Businesses use percentages to understand inventory production and market increases. Consumers use percentages in stores that advertise sale items at 25 and 50 per cent off, for example. Knowing some basics and tricks to calculating percentages is a fundamental math skill.

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Basic Percentage Trick

For starters, it can help to picture percentages as concrete items. For example, when you have 100 per cent of a number, you could imagine that 100 per cent of 70 apples equals 70 apples. If you want to calculate 50 per cent of 70 apples, however, then you must multiply 70 by the fraction 50/100. This fraction is used because 50 per cent equals 50 out of 100. So one trick is to write out the equation (50/100) x 70 and solve to find that 50 per cent of 70 is 35 apples, or exactly half of 70.

Word Problem Percentages

Word problems also are useful, for example if a farmer buys 75 per cent of 70 apples at the market, how many apples are left for other farmers? The long way is to calculate the percentage by multiplying (75/100) by 70, then subtracting the answer 52.5 from 70 to get 17.5 apples as the remainder. A shorter route, however, is to calculate the percentage left from 70 apples. Since the farmer takes 75 per cent, subtract that percentage from 100 to get 25 per cent. So, 25 per cent of apples is left over. Now, multiply 25/100 by 70 to get the same answer as before, or 17.5 apples are left for the other farmers.

Finding Percentages

A shipment of apples arrives, but 40 of the 80 apples are rotten. The trick to finding the percentage of the rotten apples is to calculate by creating a fraction from 40 and 80. Since it is 40 out of 80 apples that are rotten, the fraction is 40/80. Cross out the zeroes in the equation to simplify the fraction to 4/8, then simplify again to get 1/2. So if 1/2 of 100 is 50, then 50 per cent of the 80 apples are rotten.

Online Calculators

If you still have trouble with calculating percentages, another trick is to use a percentage calculator. Plenty of percentage calculators exist online. Math.com allows you to calculate percentages in a number of different ways. Marshu.com also has a percentage calculator that finds the per cent increase in totals.